Mass spectrometry (MS) is an analytical technique for determining the elemental composition of unknown sample substances that has both quantitative and qualitative applications. For example, MS is useful for identifying unknown substances, determining the isotopic composition of elements in a molecule, and determining the structure of a particular substance by observing its fragmentation, as well as for quantifying the amount of a particular substance in the sample. Mass spectrometers typically operate by ionizing a test sample using one of many different available methods to form a stream of positively charged particles, i.e. an ion stream. The ion stream is then subjected to mass differentiation (in time or space) to separate different particle populations in the ion stream according to mass-to-charge (m/z) ratio. A downstream mass analyzer can detect the intensities of the mass-differentiated particle populations in order to compute analytical data of interest, e.g., the relative concentrations of the different particle's populations, mass-to-charge ratios of product or fragment ions, and other potentially useful analytical data.
In mass spectrometry, ions of interest (“analyte ions”) can coexist in the ion stream with other unwanted ion populations (“interferer ions”) that have substantially the same nominal m/z ratio as the analyte ions. In some cases, the m/z ratio of the interferer ions, though not identical, is close enough to the m/z ratio of the analyte ions that it falls within the resolution of the mass analyzer, thereby making the mass analyzer unable to distinguish the two types of ions. Improving the resolution of the mass analyzer is one approach to dealing with this type of interference (commonly referred to as “isobaric” or “spectral interference”). Higher resolution mass analyzers, however, tend to have slower extraction rates and lose significant ion signal as the mass resolution increases. Furthermore, limits on the achievable resolution may also be encountered.
Inductively coupled plasma mass spectrometry (ICP-MS) has been gaining favor with laboratories around the world as the instrument of choice for performing trace elemental analysis. ICP-MS instrument detection limits are at or below the single part-per-billion (ppb) level for much of the periodic table, the analytical working range is nine orders of magnitude, productivity is superior to other techniques, and isotopic analysis can be readily achieved. Most analyses performed on ICP-MS instrumentation are quantitative; however, ICP-MS can perform semi-quantitative and qualitative analysis as well, identifying and/or quantifying an unknown analyte by detecting and/or quantifying any of 80 detectable, differentiable elements, for example.
In ICP-MS analysis, samples are typically introduced into an argon plasma as aerosol droplets. The plasma dries the aerosol, dissociates the molecules, then removes an electron from the components, thereby forming singly-charged ions, which are directed into a mass filtering device known as a mass spectrometer. Most commercial ICP-MS systems employ a quadrupole mass spectrometer which rapidly scans the mass range. At any given time, only one mass-to-charge (m/z) ratio will be allowed to pass through the mass spectrometer from the entrance to the exit. Upon exiting the mass spectrometer, ions strike the first dynode of an electron multiplier, which serves as a detector. The impact of the ions releases a cascade of electrons, which are amplified until they become a measurable pulse. The intensities of the measured pulses are compared to standards, which make up a calibration curve for a particular element, to determine the concentration of that element in the sample.
Most ICP-MS instruments include the following components: a sample introduction system composed of a nebulizer and a spray chamber; an ICP torch and an RF coil for generating the argon plasma that serves as the ion source; an interface that links the atmospheric pressure ICP ion source to a high vacuum mass spectrometer; a vacuum system that provides high vacuum for ion optics, quadrupole, and detector; a collision/reaction cell that precedes the mass spectrometer and is used to remove interferences that can degrade achievable detection limits; ion optics that guide the desired ions into the quadrupole while assuring that neutral species and photons are discarded from the ion beam; a mass spectrometer that acts as a mass filter to sort ions by their mass-to-charge ratio (m/z); a detector that counts individual ions exiting the quadrupole; and a data handling and system controller that controls aspects of instrument control and data handling for use in obtaining final concentration results.
In an inductively coupled plasma ion source, the end of a torch comprising three concentric tubes, typically quartz, is placed into an induction coil supplied with a radio-frequency electric current. A flow of argon gas can then be introduced between the two outermost tubes of the torch, where the argon atoms can interact with the radio-frequency magnetic field of the induction coil to free electrons from the argon atoms. This action produces a high-temperature (perhaps 10,000K) plasma comprised mostly of argon atoms with a small fraction of argon ions and free electrons. The analyte sample is then passed through the argon plasma, for example, as a nebulized mist of liquid. Droplets of the nebulized sample evaporate, with any solids dissolved in the liquid being broken down into atoms and, due to the extremely high temperatures in the plasma, stripped of their most loosely-bound electron to form a singly charged ion.
Thus, the ion stream generated by an ICP ion source often contains, in addition to the analyte ions of interest, a large concentration of argon and argon-based spectral interference ions. For example, some of the more common spectral interference ions include Ar+, ArO+, Ar2+, ArCl+, ArH+, and MAr+ (where M denotes the matrix metal in which the sample was suspended for ionization), and also may include other spectral interference ions such as N2+, CO+, ClO+, MO+, and the like. Other types of ion sources, including glow discharge and electrospray ion sources, may also produce non-negligible concentrations of spectral interference ions.
Aside from using high-resolution mass analyzers to distinguish between analyte and interferer ions, another way of mitigating the effects of spectral interferences in the ion stream is to selectively eliminate the interferer ions upstream of the mass analysis stage. According to one approach, the ion stream can be passed through a pressurized cell, referred to as a reaction cell or a dynamic reaction cell (DRC) if a quadrupole is used as the cell, which is filled with a selected gas that is reactive with the unwanted interferer ions, while remaining substantially inert toward the analyte ions. As the ion stream collides with the reactive gas in the reaction cell, the interferer ions form product ions that no longer have substantially the same or similar mass-to-charge (m/z) ratio as the analyte ions. In an alternative approach, the gas is reactive with the analyte ions, while remaining substantially inert toward the unwanted interferer ions. For example, the analyte ions may selectively form product ions with the reactive gas that no longer have substantially the same mass-to-charge (m/z) ratio as the unwanted interferer ions. This is referred to as the “mass shift” approach, where the analyte ion is detected as its corresponding product ion at a higher, interference-free m/z ratio.
If the mass-to-charge (m/z) ratio of the product ion substantially differs from that of the analyte, then conventional mass filtering can be applied to the cell to eliminate the product interferer ions without significant disruption of the flow of analyte ions. Thus, the ion stream can be subjected to a band pass mass filter to transmit only the analyte ions to the mass analysis stage in significant proportions. Use of a reaction cell, such as a DRC, to eliminate interferer ions is described, for example, in U.S. Pat. Nos. 6,140,638; 6,627,912; and 6,875,618, the entire contents of which are incorporated herein by reference.
In general, the reaction cell can provide extremely low detection limits, even on the order of parts or subparts per trillion depending on the analyte of interest. For the same isotope, certain limitations or constraints are imposed upon the reaction cell. For one thing, because the reactive gas must be reactive only with the interferer ion and not with the analyte (or only with the analyte and not with the interferer ion), the reaction cell is sensitive to the analyte ion of interest. Different reactive gases may need to be employed for different analytes. In other cases, there may be no known suitable reactive gas for a particular analyte. In general, it may not be possible to use a single reactive gas to address all spectral interferences.
Selenium (Se) is an essential element to human health at low levels, typically between 20 and 80 micro-gram per liter (μg/L), but becomes toxic at elevated levels. Furthermore, selenium exists in different forms that affect its toxicity and bioavailability. There is a benefit in determining the concentration of selenium in various forms, particularly at very low levels of concentration.
ICP-MS has been used to detect and quantify selenium species and selenium-containing compounds in samples. However, with conventional quadrupole ICP-MS, the most abundant isotope of selenium, 80Se, cannot be used for the determination due to the interfering 40Ar2+ dimer from the argon plasma which occurs at the same mass-to-charge ratio (m/z). As a result, selenium is normally determined using the 82Se isotope, which is only 8.7% abundant. This limits the detection capability for selenium to the 0.5-10 μg/L range using conventional ICP-MS.
Improved selenium detection has been achieved with a reaction cell chamber to eliminate the Ar2+ background using methane (CH4), for example, as the reaction gas. However, the use of methane as a reaction gas in a reaction cell is ineffective for analysis of certain complex samples due to the resulting complex gas phase chemistry and side reactions, which create new interference ions for selenium.
Another element for which high detection accuracy is often required is silicon (Si), which is a contaminant of petroleum products such as diesel fuel, naphtha, toluene, gasoline, and the like. For example, in the petrochemical industry, there is a strong desire to measure silicon in naphtha, which is a class of organic compound that can be analyzed at ten times (10×) dilution in xylene or another solvent. Analysis of such samples having complex organic matrices is challenging because of the nature of the matrix—high viscosity samples which must be diluted in volatile solvents.
ICP-MS has been used to detect and quantify silicon species in samples with complex organic matrices. However, detection of the major isotope of silicon (m/z 28, 92.2% abundance) suffers from polyatomic interferences, namely, N2+ and CO+. In organic solvents such as xylene, for example, conventional ICP-MS detects a CO+ signal much higher than normal due to the excess carbon present in the matrix.
Improved silicon (28Si) detection in aqueous solutions has been achieved with a reaction cell chamber, such as a DRC, to eliminate interfering ionic species by using ammonia (NH3) as the reaction gas. However, while ammonia may be effective for detection of silicon in aqueous solutions, ammonia is not as effective for detection of silicon in organic matrices, where interfering species such as CO+ are dominant.
As an alternative to the reaction cell approach, collision cell operation may be employed where the ion stream is collided inside the pressurized cell with a substantially inert gas. This is sometimes referred to as kinetic energy discrimination (KED). Here, both the analyte and interferer ions are collided with the inert gas, causing an average loss of kinetic energy in the ions. The amount of kinetic energy lost due to the collisions is related to the collisional cross-section of the ions, which is related to the elemental composition of the ion. Polyatomic ions (also known as molecular ions) composed of two or more bonded atoms tend to have a larger collisional cross-section than do monatomic ions, which are composed only of a single charged atom. This is due to the atomic spacing between the two or more bonded atoms in the polyatomic ion. Consequently, the inert gas can collide preferentially with the polyatomic atoms to cause, on average, a greater loss of kinetic energy than will be seen in monatomic atoms of the same m/z ratio. A suitable energy barrier established at the downstream end of the collision cell can then trap a significant portion of the polyatomic interferer and prevent transmission to the downstream mass analyzer.
Relative to reaction cell operation, collision cell operation has the benefit of being generally more versatile and simpler to operate, because the choice of inert gas does not substantially depend on the particular interferer and/or analyte ions of interest. A single inert gas, which is often helium, can effectively remove many different polyatomic interferences of different m/z ratios, so long as the relative collisional cross-sections of the interferer and analyte ions are as described above. At the same time, certain drawbacks are associated with collision cell operation. In particular, collision cell operation can have lower ion sensitivity than reaction cell operation because some of the reduced energy analyte ions will be trapped, along with the interferer ions, and prevented from reaching the mass analysis quadrupole. The same low levels of ions (e.g. parts and subparts per trillion) can therefore not be detected using collision cell operation. It has been observed that the detection limits can be 10 to 1000 times worse using collision cell operation relative to reaction cell operation. This is the case for detection of selenium and silicon via collision cell operation—sensitivities are poor.
Thus, there is a need for improved methods and systems for the detection of selenium in samples, particularly at low levels. There is also a need for improved methods and systems for the detection of silicon in samples, particularly in samples with complex organic matrices, such as petroleum products.